Bags of ready-mix concrete beside a mixer

Reference

Concrete bag yields and mix ratios

Every figure you need to turn a measured volume into an order: what each bag size yields, how many bags make a cubic yard, how far a yard spreads at each slab thickness, and the mix proportions behind each strength class.

How many cubic feet is an 80 lb bag of concrete?

An 80 lb bag of ready-mix yields about 0.60 cubic feet of finished concrete. The yield is lower than the dry weight suggests because the mix compacts as water is added. A cubic yard is 27 cubic feet, so bag counts follow directly.

Bag size	Yield	Bags per cubic yard	Bags per cubic metre
40 lb (18 kg)	0.30 cu ft	90	118
50 lb (23 kg)	0.375 cu ft	72	95
60 lb (27 kg)	0.45 cu ft	60	79
80 lb (36 kg)	0.60 cu ft	45	59

Bag counts are rounded up: 45 eighty-pound bags give 27.0 cubic feet exactly, but nobody sells a partial bag, and a pour that runs short mid-way cannot be rescued.

How much is one yard of concrete, in coverage?

One cubic yard covers a fixed area that depends only on thickness. These are the numbers that decide whether a job is a bag job or a truck job.

Slab thickness	Area per cubic yard	Typical use
2 in (50 mm)	162 sq ft	Topping and repair layers
3 in (75 mm)	108 sq ft	Footpaths and light pads
4 in (100 mm)	81 sq ft	Patios, shed bases, sidewalks
5 in (125 mm)	65 sq ft	Driveways for cars
6 in (150 mm)	54 sq ft	Garage floors, heavy vehicles
8 in (200 mm)	40 sq ft	Structural and load-bearing slabs

Concrete mix ratio by strength and application

Ratios are cement : sand : coarse aggregate, measured by volume. Strengths are the figure a mix reaches at 28 days with a water-cement ratio of about 0.50 and reasonable compaction. Site-mixed concrete rarely beats the same nominal mix from a plant, so treat these as targets rather than guarantees.

Mix ratio	Strength at 28 days	Where it is used
1 : 3 : 6	2 000 psi (14 MPa)	Blinding layers, mass fill, kerb bedding
1 : 3 : 5	2 500 psi (17 MPa)	Footpaths and other non-structural flatwork
1 : 2 : 4	3 000 psi (21 MPa)	Patios, sidewalks and general ground slabs
1 : 2 : 3	3 500–4 000 psi (24–28 MPa)	Driveways, garage floors, strip footings
1 : 1.5 : 3	4 500 psi (31 MPa)	Structural slabs, columns, heavy loading

Bagged ready-mix already contains cement, sand and aggregate in a fixed proportion — usually equivalent to a 1 : 2 : 3 mix at around 4 000 psi. Mix ratios matter when you are batching from separate materials, not when you are opening bags.

The formula for figuring concrete

Four steps, in the order they are actually done. Thickness is the step people get wrong, because it is quoted in inches while everything else is in feet.

volume (cu ft) = length ft × width ft × (thickness in ÷ 12)
cubic yards = volume ÷ 27
ordered = cubic yards × waste factor (1.05 to 1.15)
bags = ⌈ (ordered × 27) ÷ bag yield ⌉

Worked example. A 12 ft × 10 ft patio at 4 inches: 12 × 10 × 0.333 = 40 cubic feet, or 1.48 cubic yards. With a 10% allowance that is 1.63 yards — 44 cubic feet — which is 74 eighty-pound bags. At that point a ready-mix truck is cheaper, faster and stronger, which is the real reason the bag table matters.

Run these numbers for your slab →

Water, waste and the two figures nobody adjusts

A water-cement ratio between 0.45 and 0.60 by weight is the usable band. Below it the mix will not compact; above it strength falls off sharply — roughly 10% of strength for every 0.05 of extra water. An 80 lb bag takes about 3 quarts, or 6 pints, of water.

Waste allowance runs 5% on solid formwork over a level, compacted sub-base, and 10–15% where the ground is soft, hand-dug or uneven. The allowance covers spillage, over-dig and the concrete left in the barrow — not a change of mind about thickness.

Put these figures to work

These numbers are the inputs every concrete calculator on the site runs on — slabs, footings, steps and the posts set in them. Each tool applies them to one part of the job.

Tools for the job

What makes this job easier

Tools and materials that suit this kind of job. These are affiliate links: if you buy through one, Renovimo may earn a commission at no extra cost to you. It never changes the figures on this page.